Comparison of AP trains recorded experimentally with those simulated using the CD model. Trains of APs show a similar baseline depolarization of the membrane potential in experimental recordings from rat EDL muscle fibers (A) and in model simulations (B) when stimulated at 6, 15, or 30 Hz. The resting membrane potential during AP trains was evaluated as the membrane potential immediately before injection of excitatory current through the current electrode. At each frequency, the depolarization of resting membrane potential (C) was similar in the experimental (open circles; mean ± SEM) and model records (solid lines). In the model (D), this depolarization resulted from K⁺ buildup in the t-system that was greatest in the innermost t-system shell (uppermost traces), and least in the outermost shell (lowest traces).